Since you asked about the algae eater first, we'll start there. If it's a chinese algae eater, Gyrinochelius aymonieri (actually from Thailand lol), then yes they will often feed on sick and dying fish, and sometimes even attack healthy ones. As they grow older, they also tend to sour considerably in disposition, not to mention that they can reach sizes up to a foot (though many on the web will try to tell you they top out at about 4-5), they are overall not good candidates for your 29 gallon. Additionally, if your pleco is a common pleco, it will also outgrow your tank. In reality, a 29 gallon is not really suitable for the majority of your fish. At MOST, a 29 is really only sufficient for 2 adult goldfish (not even that imo), and the rest will eventually have to go or be re-homed. You said you're unsure of why your oranda passed, but without any tank readings, (ammonia, nitrate, nitrite etc...) we can't tell you with certainty either, though it seems like it was due to inappropriate, stocking levels and decisions. Even with your weekly w/c schedule, I'd bet that levels remain in dangerous ranges for a large amount of the time. In the end, my best advice would be to return all of your fish, and re stock with small tropicals that are more suited to life in a 29.